Automation enables you to tell Reaper to change settings like volume, pan or parameters of FX, then each time the project plays back, Reaper can repeat those changes for you automatically. In a mixing context, this is most often what breathes life into the song and makes the mix sound/feel less static.

Every automation move involves what Reaper calls envelope types and envelope points. Each specific parameter or control that you automate is an envelope type, for example, volume is an envelope type, pan is a different envelope type. Each instruction you give Reaper that's related to volume, pan or any other type of envelope is an envelope point. It's possible to have as many envelope types and points active as you need on any given track to get it to do what you want. Let's look at a workflow for a volume example.

1. Select a track that you want to modify the volume of automatically, then hit Control+Alt+V to show the volume envelope. Thanks to OSARA, your screen reader will report "showed volume envelope". The envelope type you want to work with always needs to be shown before you can select it and start dropping points.
2. Hit Alt+L to select the volume envelope you've just shown. Note that if you've got multiple envelope types active on a track, then Alt+L will select the next envelope, Alt+Shift+L will select the previous one. It's a cycle (as in it wraps around), so retaining both of those keystrokes isn't essential until you've got loads of different envelope types visible on each track, just saves you a few seconds now and then.
3. Now move to a place where you want a volume change to start, and hit Alt+Shift+E to drop an envelope point. A new window will open with the current track volume pre-filled in for you. Most of the time, this is what you'll want to use for the first point.
4. Scooch forward a bit to the time when the volume change should complete, hit Alt+Shift+E again to place another point, and this time, type the new volume that you want the track to reach. While you're experimenting, I'd recommend using a volume that's somewhere around 10dB quieter so you'll definitely be able to perceive the change.
5. Back up a few bars and play that in context. You should find that over the duration between those two points you've placed, Reaper will ramp the volume up or turn it down automatically (depending on whether the second point you placed was louder or quieter than the first one).
So, what you can take from this is that often times, an automation move will require a minimum of two points, the value to start at and the value to end at.

Here's a few gotchas to watch out for:

1. Although you've only placed two points in that example, if you now hit Alt+J to go to the previous envelope point or Alt+K to go to the next point, you'll find that there's actually 3 points placed on the track. The additional point is one that Reaper automatically places at bar 1, beat 1 as soon as you select the volume envelope. This point is necessary for it to know what volume to start playing the track at.
2. OSARA can be a bit finicky about selection of envelope types, so sometimes you might need to give it a kick to remind it which envelope type you're working with. In my experience, visibility of the envelope never gets lost, so if you find it's misbehaving, the thing to do is to give your Up/Down arrows a quick wiggle to re-select the relevant track, then either Alt+L or Alt+Shift+L to re-select the envelope type that you're working with. Hopefully we'll get that smoothed out at some point.

All of the above applies to every envelope type, and there are scores of Reaper's controls that can be shown as envelopes. Most of the above workflow also holds up irrespective of which envelope type you're automating. For example, if you wanted to automate pan instead of volume, you'd just hit Control+Alt+P to show the pan envelope instead, and follow the above through from there to work with pan. To show additional envelope types that aren't on the keymap, use the actions list.

Last thing for now, you can also automate FX parameters. Let's look at an example of how to get that going.

1. Pop an effect on a track, a reverb for example.
2. In either the FX window or the OSARA Parameters window, give the control you want to automate a wiggle (let's say room size for this example), then close that window.
3. Back in the track list, with the relevant track still selected, hit Control+Alt+L to show the last-touched parameter as an envelope. As with volume and pan above, your screen reader will report that a new envelope type has been shown.
4. Hit Alt+L to select the envelope you've just shown, and you know what to do from here, Alt+Shift+E to start dropping points.

Hope that's helpful! If you're keen to support the time it takes to write stuff like this up, donations of any amount sent to https://paypal.me/ScottChesworth are much appreciated.